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Figure 6.25 The basic PIC18 architecture use for referencing instruction operation.
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The PIC18 processor has a number of FSR index registers with FSR before and after increment, after decrement, and the ability to access data relative to the FSR. This will make compiler development much simpler for the PIC18 than for any of the other PIC microcontrollers. As I mentioned earlier, instruction, formatting, and execution are similar in the PIC18, with the major difference being the direct register addressing options. In the PIC18, there are a number of word instructions that allow goto and call instructions throughout the entire 2-MB maximum program memory space, as well as the ability to move register contents directly within the memory space. The PIC18 s ALU has been enhanced compared with the other PIC microcontrollers by the inclusion of add with carry and true subtract with borrow instructions. The true subtract with borrow instruction works as you would expect instead of the typical reversed PIC microcontroller instruction, that is,
subwfb Reg, Dest ; Dest = Reg WREG B
The PIC18 offers the
subfwb Reg, Dest ; Dest = WREG Reg B
which works in a more traditional manner than the standard PIC microcontroller subtraction instructions. This new subtraction instruction will make the transition to PIC18 microcontrollers easier for people familiar with other computer processors. The PIC18 has an 8 8 multiplier that runs in a single-cycle instruction. This feature multiplies the contents of the WREG register by a constant or the contents of a register and places the 16-bit result in the PRODH and PRODL registers. The multiplier allows the implementation of some basic DSP algorithms in your applications. Remembering a trick from high school, you can use the 8-bit multiply capability to multiply two 16-bit numbers together. When you were learning basic algebra and multiplying two, two value expressions together, you were taught to multiply the two rst values together, followed by the outside, inside, and last. The acronym you were given was FOIL, and it could be described as (A B) (C D) AC AD BC BD
By breaking a 16-bit number into 2 bytes and recognizing that the high byte is multiplied by 0x0100, A and B can be written as A For A (AH 0x0100) AL and B (BH 0x0100) BL
B, the numbers can be broken up into two parts and then FOILed: AxB (AH BH 0x0100 0x0100 AL) (BH 0x0100 BL AL BL) BH 0x0100
0x0100
AH 0x0100 AL BL
THE MICROCHIP PIC MCU PROCESSOR ARCHITECTURE
Knowing that multiplying by 0x0100 is the same as shifting up by 1 byte (or by 8 bits), the two 16-bit variables, A and B, can be multiplied together into the 32-bit product using the code
Product = MUL(AL, BL); TProduct = MUL(AL, BH) << 8; Product = Product TProduct; TProduct = MUL(AH, BL) << 8; Product = Product TProduct; TProduct = (MUL(AH, BH) << 8) << 8; Product = Product TProduct;
This algorithm can be converted very easily to assembly language.
Register access and bank addressing
The PIC18 register architecture is probably the nicest of the four PIC microcontroller architecture families. While there is still banking, the variable placement rules I discuss elsewhere in this book still apply, with the ability to access key variables directly, as well as the I/O hardware registers [called the special function registers (SFRs) in the PIC18]. In the applications that I have done for the PIC18, I have found that I have had to think the least about variable placement and hardware register accessing. The PIC18 can access up to 4,096 eight-bit registers that are available in a contiguous memory space. Twelve address bits are used to access each address within the register map space shown in Fig. 6.26. While there are still register banks, the le registers from
Register Map Bank 0 Bank 1
0x01FF 0x0200 0x0000 0x007F 0x0080 0x00FF 0x0100
Access RAM GPR GPR GPR GPR Access Bank Access RAM SFR a=0
0x000 0x07F 0x080 0x0FF
Bank 2 Bank 3
0x02FF 0x0300 0x03FF
0x0E00
Bank 14 Bank 15
0x0EFF 0x0F00 0x0F7F 0x0F80 0x0FFF
GPR GPR SFR a=1
Figure 6.26 The PIC18 register organization allows for sixteen 256-byte register banks or a single bank combining 128 le registers and 238 SFRs.
ARCHITECTURE DIFFERENCES
one bank to the next can be accessed simply by incrementing one of the three FSR registers instead of the redirection of the FSR register into the next bank that is required in the other PIC microcontrollers. The FSR registers either can be loaded with a full 12-bit address using the lfsr instruction or can be accessed directly by the application. To access a register in a speci c bank directly, the PIC18 s bank select register (BSR) must be set to the bank in which the register is located. The BSR contains the upper 4 bits of the register s address, with the lower 8 bits explicitly speci ed within the instruction. The direct address is calculated using the formula Address (BSR 8) direct address
To simplify directly accessing variables, the rst 128 addresses are combined with the second 128 addresses as shown in Fig. 6.26 to make up the access bank. This bank allows direct addressing of the special function registers (SFR) in the PIC microcontroller, as well as a collection of variables, without having to worry about the BSR register. Practically speaking, the access bank means that for the rst time in the PIC microcontroller, you will be able to access most, if not all, of the registers required in an application without having to specify a bank or use a special instruction (such as tris and option). This greatly simpli es the task of developing PIC18 applications and avoids some of the more dif cult aspects of learning how to program the PIC microcontroller in assembly language how to access data in different banks. The hardware I/O registers (SFR) are located in the last 128 addresses of the register space. This may seem limiting, but remember that this is more dedicated hardware register space than is available in any of the other PIC microcontrollers. The index register operation of the PIC18 is very well organized and will make it much easier for compiler writers to create PIC18 compilers than for other PIC microcontrollers. Along with the three 12-bit-long FSR registers, when data is accessed, it can result in the FSR being incremented before or after the data access, decremented after, or access to the address of the FSR contents added to the contents of the w register. A speci c access option is selected by accessing different INDF register address. Table 6.9 lists the different INDF registers and their options concerning their respective FSR registers.
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